JG Electronics

PreAmp (DI box) and Single Ended Class A Power Amp EL84

Vox AC4 / Fender Rhodes preamp based project.

Guitar Preamp (DI box) with tone control and line level out and a single ended class A power amp with EL84.

The guitar preamp (DI box) is based on the Fender Rhodes Piano PreAmp. The dual operational amplifiers used in this project  are low noise integrated circuits. Use a stabilized power supply with voltages not lower than +12 and -12V and not higher than +15V and -15V. The input circuit is perfect for an electric guitar, the active tone control works perfect and the amplifier produces harmonics which results in a warm sound.

The output consists of two separate buffered outputs at line level. This preamp you can use as a DI Box after your pedals with one output connected to your guitar amp and one output to a mixer. You can use it to play with your guitar over your home stereo installation, be careful not to damage your speakers with to loud guitar sounds. A speaker for a guitar is build different to handle the loud single guitar tones.  To download the schematic of the PreAmp click here

Update: Power Supply pin2 of R14  should connect to the ADJ pin of IC3 LM317T and pin2 of R16 should connect to the ADJ pin of IC4 LM317T.

It's easier to use two AC/DC PCB Power Modules: input 90 ~265VAC, output 12VDC.   i.e. Part No. VTX-214-001-112

Front View Power Amp
Top View Power Amp
SideView Power Amp
Side View Power Amp with EL84
Front View Complete Power Amp
PreAmp, Power Amp, Speaker and Power Distribution Box
Guitar PreAmp (DI box)
Guitar PreAmp (DI box)
Philips PE4820

© JG Electronics

The single ended class A  amplifier with 1xEL84 is based on the VOX AC4 amplifier. Output transformer NOS from Amroh model 5055. If you don't have an output transformer you can buy from hammondmfg.com model 1750B or from mercurymagnetics.com the output transformer for the AC4.

Power supply, I use a regulated power supply for testing and experimenting, a Philips PE4820 that I received as a gift because it was never used any more. The Va and Vg2  for the EL84 in class A is 250 Volt. The the filament voltage Vf is 6,3 Volt AC or DC, the filament current If is 0.76 Amp. To download the schematic of the power amp with EL84 click here